Extra posting) Q. What's the meaning of yellow ribbon?

By Henbro117 (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

I first learned a yellow ribbon in Sewol ferry memorial campaign.
The yellow ribbons are always with the slogan, "We will not forget."
So I think the yellow ribbon means "We will not forget."